Imo Smeargle should be higher than it is, since it has literally every move in the game, giving it quite a large range of niches. It's something of a Jack-of-all-Trades, although there are some things it excels at. One cool thing is the ability to Imprison EVERYTHING (not at once, but if you forget to sub for it it's gg, and even if you do sub for it you can get screwed). Also it gets every OHKO move, which can be fun in combos, in addition to being the only pokemon that anyone has (until somebody catches an Articuno) with both an OHKO move and Lock-On or Mind Reader. This is on top of getting every possible shenanigan, such as Encore, Crafty Shield, Fake Out, Torment, etc., for major sub eating.
Another fun thing to note is that there are only 10 FE pokemon that can successfully Taunt a well-trained Smeargle with 3 subs- (Mega) Gengar, Froslass, +Spe Chandelure, Mismagius, Kitsunoh, Alakazam, Crobat, +Spe Glalie, Mienshao, and Sawk. (Note that Smeargle can stop Taunts with Crafty Shield, P/E, Magic Coat, Quick Guard (If Prankster), Fake Out (Assuming no ghost-type or Inner Focus, and only once), faster DE, faster Taunt, faster Imprison, faster Sleep-inducing moves, and various hax-based methods. Or, you know, a Mental Herb)
It is held back by being super squishy and having zero offensive presence outside of OHKO combos and Berserk Gene, though. Counter, Mirror Coat, Bide, and Metal Burst (when slower) counteract this somewhat but not fully. I'm thinking around a B- or C+ or so for Smeargle, although you could really put it anywhere because the usefulness of Smeargle is incredibly variable based on how many moves it has (to a MUCH greater extent than other pokemon). Also it's really difficult to train a Smeargle to competitive levels, you need a lot of moves, and UC dumping Smeargle is really inefficient.